Carbon Nanotube Muscles Strong as Diamond, Flexible as Rubber

For the next installment of the Terminator franchise, Hollywood might skip the polymimetic liquid alloys — they’re so 2003 — and turn to the laboratory of Ray Baughman, who has created a next-generation muscle from carbon nanotubes.

Baughman and his colleagues have produced a formulation that’s stronger than steel, as light as air and more flexible than rubber — a truly 21st century muscle. It could be used to make artificial limbs, "smart" skins, shape-changing structures, ultra-strong robots and — in the immediate future — highly-efficient solar cells.

"We can generate about 30 times the force per unit area of natural muscle," said Baughman, director of the NanoTech Institute at the University of Texas at Dallas.

Carbon nanotubes have fascinated material scientists since the early 1990s, when researchers started to explore the ultra-light, ultra-strong cylindrical molecules. Though bulk manufacturing difficulties have slowed the development of commercial applications, carbon nanotubes are already used in bicycle components, and in prototypes of airplanes, bulletproof clothing, transistors, and ropes that might someday be used to tether a space elevator. (On a historical note, carbon nanotube-infused steel was used to made Damascus blades, renowned as history’s sharpest swords, though the technique has been lost.)

Baughman became interested in carbon nanotubes while designing artificial muscles from energy-conducting polymers. He figured he could do the job better with linked carbon nanotubes. First he made haphazard tangles of fibers activated by charged liquids. Then he experimented with more structurally-consistent configurations, and other methods of delivering the charge.

His latest muscle, described Thursday in Science, is made from bundles of vertically aligned nanotubes that respond directly to electricity. Lengthwise, the muscle can expand and contract with tremendous speed; from side-to-side, it’s super-stiff. Its possibilities may only be limited by the imaginations of engineers.

"This apparently unprecedented degree of anisotropy" — direction-dependent physical properties — "is akin to having diamond-like behavior in one direction, and rubber-like behavior in the others," wrote John Madden, a University of British Columbia material scientist, in an accompanying commentary.

Baughman’s muscles rely on the tendency of an electric charge to make carbon nanotube fibers repel or attract each other, depending on their configuration.

Natural muscles, said Baughman, contract at a maximum rate of 10
percent per second. In the same amount of time, his latest nanotube sheaths can contract by 40,000 percent. Because it responds to an electrical current rather than ion movement in electrolytic liquids, it’s far more efficient than his old formulations.

The nanotube bundles retain their properties at temperatures ranging from the -320 degree Fahrenheit of liquid nitrogen to the 2800 degree Fahrenheit melting point of iron.

The first applications, said Baughman, will likely be as wrappers for solar cells, with nanotubes conducting electricity and rapidly changing shape in order to produce optimally light-sensitive arrangements.

"We’ve characterized the activity and performance," he said. "Now we want to use them."